A Question of Balance: Optimizing Belowground Carbon Deposition by Limiting Crop Nutrient Levels

Project Overview

Building soil organic carbon (SOC) is an important management practice in organic agriculture. SOC improves soil structure and water retention, supports healthy plant-microbial interactions, and can serve as a tool to mitigate climate change by sequestering atmospheric carbon. Management practices including cover cropping, organic compost/fertilizer application, and reduced tillage are known to increase SOC, but there are also ways in which adjusting crop resource availability can induce a physiological shift in plant C allocation, resulting in the deposition of carbon-rich exudates into soils and accumulation of SOC.

This study reviews the existing literature on belowground C deposition and posits several ways in which farmers might tap into the physiological responses of crops to enhance SOC.

Farmer Takeaways

  • Maintaining N, P, and water availability at levels slightly less than optimal for aboveground crop growth can induce the deposition of surplus photosynthates belowground and contribute to the accumulation of stable, mineral-associated soil organic carbon. 
  • ‘Optimal’ resource levels for inducing surplus photosynthate deposition belowground will likely vary by crop type, soil type, land-use history, and climatic conditions (i.e. temperature, precipitation). Additional studies are needed to elucidate site- and crop-specific C responses to resource deficiencies.

Project Objectives and Approach

Reviewing the Literature

Several studies assessing belowground C deposition and SOM accumulation across different cropping systems were reviewed. 

Proposing Future Studies

In order to further understand the potential of these mechanisms to enhance labile, stable, short- and long-term SOM, several studies have been proposed. The researchers propose that studies be conducted that simultaneously measure C fixation, aboveground and belowground biomass production, and root exudation for a variety of common crops maintained at set levels of N, P, water, and temperature.

Key Findings

Carefully managing the availability of N, P, and water to maintain a deficiency for aboveground plant growth may result in the deposition of surplus photosynthate belowground as root exudates

  • Withholding one of the resources that limits aboveground growth in order to enable the deposition of surplus C to the soil may result in slightly lower crop yields than for systems with intense fertilization and irrigation. 
  • Prioritizing SOC improvements should not be undervalued; enhanced SOC levels and plant-microbial interactions will improve soil structure and fertility, water infiltration and retention, and nutrient cycling, all of which can provide long-term agricultural resiliency during times of uncertainty risk.

Incorporating leguminous plants may enhance the accumulation of mineral-associated soil organic matter (MAOM) by providing the N necessary for the generation of soil microbial biomass

  • Accumulating soil microbial biomass, which is an important component of stable, mineral-associated soil organic matter (MAOM), requires substantial quantities of both nitrogen and carbon. Incorporating nitrogen-fixing leguminous plants into cropping systems can provide both of these nutrients and stimulate production of new microbial biomass, necromass, and SOM. 

Maintaining grass in the portion of the growth phase in which leaves generate surplus carbon may enhance SOC 

  • In unfertilized pastures, grass likely generates surplus photosynthate later during the active growth phase (Phase 2), when leaf biomass has recovered from grazing and photosynthesis rates are high. 
  • By prohibiting grazing during the recovery period (Phase 1) and allowing grazing prior to the onset of Phase 3, it might be possible to maximize both aboveground biomass and belowground C flux.

Resources

Prescott, C. E., Rui, Y., Cotrufo, M. F., & Grayston, S. J. (2021). Managing plant surplus carbon to generate soil organic matter in regenerative agriculture. Journal of Soil and Water Conservation, 76(6), 99A-104A.

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Location

British Columbia, Canada

Collaborators

Yichao Rui, Rodale Institute

Francesca Cotrufo, Colorado State University

Sue Grayston, University of British Columbia, Vancouver

by research